SINGAPORE, March 25 (Reuters) - India has pushed back by a year its plan for coal-fired power plants to lower output ‌when solar generation is high, as regulators work out how to compensate for the higher costs of retrofitting entailed, documents reviewed by Reuters show.

Analysts say lack of flexible generation of coal power as India expands renewable capacity threatens to waste green investments, swell compensation ​costs and boost emissions from greater coal use that could otherwise have been avoided.

The move comes ​at a time when the world's second largest user of coal is curbing solar output for ⁠lack of dedicated transmission lines, while coal-fired capacity wrestles with operational constraints.

Solar generators told to cut output as ​India's coal plants could not ramp down could get compensation of as much as $76 million for the eight months ​ended December, energy think-tank Ember estimates, a cost that will be passed on to consumers.

Government officials blamed the delay of a year on the absence of rules to compensate coal plants for higher costs of maintenance and retrofitting needed to cut the minimum ​use rate to 40% from 55%, the minutes of a January 16 meeting showed.

Retrofitting coal plants would swell tariffs ​by as little as 0.28 rupees to 0.60 rupees per kilowatt-hour, versus 5.76 rupees to 6.04 rupees for battery storage, making ‌flexible ⁠coal at least 10 times cheaper, the Central Electricity Authority (CEA) said at the meeting.

India's power ministry did not respond to requests seeking comment.

CHINA'S MORE AMBITIOUS COAL PHASEDOWN

Unveiled in 2023 with its first phase making slow progress, the plan is less ambitious than efforts by China, which last year cut the minimum coal plant utilisation rate to a ​range of 25% to 40% ​from 50% to 60% ⁠to boost renewables use.

Indian state coal plant operator NTPC warned the January meeting against "accelerated wear and tear of critical equipment" that stems from operating at minimum loads of ​40%.

NTPC urged "detailed studies" on ways to cut use to avoid such damage, adding that ​its new project ⁠contracts included the 40% requirement.

But CEA officials responded that other countries' coal plants running at lower levels of output have been shown to operate safely if properly retrofitted.

The federal regulator has yet to approve the higher maintenance costs proposed ⁠by CEA, ​citing lack of operational data, the agency's presentation showed.

Senior officials of ​the federal power ministry, CEA, the federal regulator, the grid operator, NTPC and industry lobby group Association of Power Producers agreed to study the ​impact of the plan based on latest cost estimates, the minutes showed.